Upgrading process of palm empty fruit bunches as alternative solid fuel: a review

Abstract

A total of 37.5 million tons of palm empty fruit bunches (EFBs) produced in Indonesia in 2018 have the potential to be used as an alternative eco-friendly solid fuel. However, a pretreatment process is necessary to increase the heating value and reduce the potassium content of EFBs. Several methods can be employed to improve the characteristics of EFBs as a solid fuel, such as drying and torrefaction to increase the heating value and the leaching process for reducing the potassium content of EFBs. The main concept of increasing the heating value is to increase the fuel content, which is carbon. Through drying, the carbon ratio can be increased by reducing the moisture content and, through torrefaction, the carbon ratio could be increased due to the decomposition of lignocellulose. A simple way to reduce the potassium content of EFBs is the leaching process. Two types of leaching treatment have been studied before: soaking and stirring treatment. This study reviews those methods to make EFBs more suitable as a solid fuel with a high heating value and less potassium content. Increasing the carbon content in EFBs is crucial for enhancing their heating value as a fuel. Drying and torrefaction decrease the moisture content and modify the chemical structure, resulting in higher carbon ratios. The leaching process effectively reduces the potassium content of EFBs and stirring treatment is more effective than immersion treatment. Torrefaction positively affects leaching by decreasing the potassium content, while hydrophobicity may hinder leaching by repelling water. The high moisture content of leached EFBs requires additional energy for evaporation during torrefaction, resulting in a lower energy density yield compared with raw EFBs. Adjustments may also be required to account for the decreased mineral content, which functions as a torrefaction catalyst in EFBs that have not been leached.